1. Field of the Invention
This invention relates to an injector useful in gas chromatography, and more particularly to a micromachined microvalve injector assembly that can inject a fixed volume of a fluid sample into a destination stream. The injector assembly also includes valving to reverse fluid flow through the injector, which facilitates sample analysis by the column backflush technique.
2. Description of the Prior Art
Gas chromatographs are well known for analysis of chemical mixtures, separation of gases, and process measurement and control. A gas chromatograph includes three main components: an analytical column which physically separates the components of a sample mixture, a detector which senses the individual components after separation, and an injector which introduces an amount of the sample into the analytical column for separation.
The detector is located at the outlet of the analytical column. The detector generates a printout showing peaks corresponding to the sample components (termed a "chromatogram"). These results are typically analyzed by comparing the sample profile with the profile obtained for a calibration fluid of known composition.
The injector introduces a "plug" of sample into the analytical column. As injection time increases, the peaks detected by the detector tend to broaden and overlap. Therefore, the plug is ideally injected over the shortest possible period time. For quantitative analysis, the injector must introduce a controlled amount of sample into the precolumn or analytical column. The most common injection techniques for gas chromatography are by syringe and by sample loop.
The well-known sample loop injector includes a two-position multiport rotary valve. The sample loop is filled with sample while a carrier gas flows to the analytical column through channels in the valve. When the valve rotor is turned to move the valve into the second position, the arrangement of valve channels is re-configured, and the sample loop becomes part of the channeling that conducts the carrier gas through the valve to the analytical column. In the second position, the carrier gas flow sweeps the sample from the sample loop. Sample thus flows in the carrier gas stream to the analytical column.
A variation on the sample loop is used in the 10S series gas chromatograph manufactured by Photovac International. Four solenoid valves are used to isolate the sample loop during filling with sample and insert the sample loop into the carrier gas stream during the sample injection event.
U.S. Pat. No. 4,474,889, entitled "Miniature Gas Chromatograph Apparatus" (issued on Oct. 2, 1984, and incorporated by reference herein), commonly owned with the present application, describes a sample injection scheme that utilizes a miniaturized injector. In addition to being smaller and more convenient to use, miniaturized injectors have reduced injection times. Accordingly, such injectors provide better resolution and facilitate high-speed chromatographic analysis.
The injector disclosed in U.S. Pat. No. 4,474,889 employs a "timed-injection" scheme, wherein the amount of injected sample depends on the period of time that an inject valve is open. However, the amount of sample that flows into the analytical column during a particular injection time decreases undesirably as the sample viscosity increases. Therefore, the reported response (peak area) of the associated gas chromatograph for a particular analyte decreases as the sample viscosity increases.
A recent disclosure, the "Fluid-Lock Fixed Volume Injector" (U.S. patent. application Ser. No. 08/158,978, filed on Nov. 30, 1993), commonly owned with the present application, is directed to a miniature injector that injects a fixed-volume of sample fluid without regard to fluid viscosity.
Neither of these disclosures is directed to a "backflush" capability. With respect to gas chromatography, "backflush" typically refers to the reversal of carrier gas flow through a "precolumn" that is connected in series to an analytical column. The "precolumn" is located between the sample injector and the analytical column. The injected sample flows first into the precolumn, then into the analytical column. In practice, after some of the sample components reach the analytical column, the flow through the precolumn is reversed or stopped. The sample components thereby isolated on the precolumn are processed separately from those sample components that had entered the analytical column.
The sample components in the precolumn are typically swept from the precolumn to a vent or to a detector, using carrier gas flowing in the direction opposite to the original sample flow direction. Analytical column backflushing is also possible. In this case, the gas chromatograph may contain only an analytical column.
Some conventional gas chromatograph injectors having backflush capability use a combination of rotary valves to achieve sample and carrier gas flow switching. Such rotary valves have the disadvantage of being difficult to miniaturize. In addition, relatively large dead volumes and slow switching times limit the performance of rotary sample valves. Furthermore, rotary valves undergo a limited number of cycles before the valves wear out due to friction at the seal interface.
The Photovac 10S series gas chromatograph uses solenoid valves to achieve precolumn backflush. Like rotary valves, solenoid valves are difficult to miniaturize. Moreover, solenoid valves can be susceptible to air diffusion and contamination by sample components. Both the entry of air into injector flow channels and valve contamination undermine gas chromatograph performance.
There is a need for a fixed-volume backflush sample injector that avoids the drawbacks of conventional injectors. In particular, a fixed-volume injector that can be miniaturized easily, is unaffected by variations in sample viscosity, and can provide the sample and carrier gas flow switching required for backflush operation would be desirable.